dispatch_add_command: Modify the logic so there's only one exit point instead of two. Only insert valid (non-NULL) values into the queue.
dispatch_add_command:
Modify the logic so there's only one exit point instead of two.
Only insert valid (non-NULL) values into the queue.

dispatch_free_command:
Ensure that item is not NULL before removing it from the queue and
dereferencing the pointer.
NULL out free'd pointers to catch any use-after-free bugs.

PR:	bin/146855
Submitted by:	gcooper
MFC after: 3 days

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